Current spacecraft and electric vehicles typically use rechargeable chemical batteries to store electrical energy. Despite decades of research, rechargeable chemical batteries continue to be large and heavy. Thus, they have a low volume energy density and a low weight energy density. In addition, chemical batteries are expensive and difficult to manufacture and maintain. Further, chemical batteries contain hazardous or toxic materials and require temperature control. Also, rechargeable chemical batteries can only provide a small percentage of their total stored energy as usable electrical energy. This means that the chemical batteries have a low depth of discharge. Chemical batteries also suffer irreversible progressive deterioration from each charge/discharge cycle, which means they have a limited cycle life.
Flywheels can eliminate many of the problems associated with rechargeable chemical batteries. Flywheels store kinetic energy in a high speed rotor and use a motor/generator to convert between electrical and mechanical energy. However, for many applications, flywheels cannot be simply used as "drop-in" replacements for chemical batteries. In particular, an inevitable by-product of using a spinning rotor is the production of angular momentum and torque. For many applications (such as spacecraft and underwater vehicles) the amount of angular momentum and torque produced as a by-product of using a flywheel for energy storage is several orders of magnitude greater than can be tolerated by the attitude control system (ACS).
A first order solution to the problem of unwanted angular momentum and torque is to arrange the energy storage flywheels as counter-rotating pairs. In the ideal case, the angular momentum and torque cancels exactly. However, any practical flywheel system will have some level of residual mismatch in such parameters as the rotor mass moment of inertia, spin speeds, rotor spin axis, and motor/generator power flow. Even if these parameters could be initially matched during manufacture, maintaining the match over time, and under changing temperature situations, as well as vibration environments, is not currently feasible.
The basic concept to using counter-rotating pairs of flywheels for energy storage alone or for energy storage and attitude control is discussed in U.S. Pat. No. 4,723,735. This patent mentions including at least two flywheels with their angular momenta balanced to produce zero net angular momentum. However, this reference does not state how the balance is achieved in practice.
In addition, as a practical manufacturing matter, pairs of counter-rotating flywheels must be matched to produce zero net torque. However, this is extremely difficult with known production methods. Thus, a large amount of testing and matching is needed to find pairs of flywheels that are properly matched to produce zero net torque. As a result, providing exact duplicates for counter-rotating pairs is very costly in both time and money. Therefore, a method and apparatus are needed which would allow the use of less exacting tolerances on the flywheel pairs yet still provide a zero net torque output from the pairs.
Therefore, as a result of the short-comings of rechargeable chemical batteries and known flywheel energy systems, the need has arisen for a method and apparatus for reducing mechanical disturbances from energy storage flywheels.